1. Field of the Invention
This invention relates generally to an apparatus and method for in-situ process monitoring and more specifically, to an apparatus and method for in-situ monitoring of chemical-mechanical planarization of semiconductor wafers.
2. Background
Planarization of the active or device surface of a substrate has become an important step in the fabrication of modern integrated circuits (ICs). Of the several methods of planarization that have been developed, Chemical Mechanical Polishing (CMP) is perhaps the most commonly used method. This popularity is due, in part, to its broad range of applicability with acceptably uniform results, relative ease of use, and low cost. However, the move to larger diameter wafers and device technologies that require constant improvement in process uniformity requires that an improved planarization system become available.
A typical CMP system uses a flat, rotating disk or platen with a pliable monolithic polishing pad mounted on its upper surface. As the disk is rotated, a slurry is deposited near the center of the polishing pad and spread outward using, at least in part, centrifugal force caused by the rotation. A wafer or substrate is then pressed, typically face down, against the working surface of the polishing pad such that the rotating polishing pad moves the slurry over the wafer's surface. In this manner, surface high spots are removed from the wafer and an essentially planar surface is achieved.
The planarization of an interlayer dielectric is one common use for CMP. As the topography of the underlying surface is not uniform, coating that surface with a dielectric film replicates or even magnifies those non-uniformities. As the surface is planarized, the high spots are removed and then the total thickness of the dielectric film is reduced to a predetermined value. Thus, the planarized dielectric film will be thinner over high points of the underlying surface than over low points of that surface. Typically, it is important to maintain a minimum dielectric thickness over each of the highest points of the underlying layer, both locally (within a die) and globally (across the wafer). Thus, uniform removal of the dielectric layer at all points of the wafer is required.
A problem with most existing CMP systems is their inability to perform in-situ thickness monitoring. As the surface of the wafer is pressed against the polishing pad during removal, typically no measurements as to the progress of the polishing can be made. Thus, wafers are either polished for fixed times, and/or periodically removed for off-line measurement. Recently, Lustig et al., U.S. Pat. No. 5,433,651 (Lustig) proposed placement of at least one viewing window in the working surface through the thickness of the polishing pad to provide access for in-situ measurement. However, a window placed in a polishing pad creates a mechanical discontinuity in the working surface each time the window passes across the surface of the wafer. A more conventional approach is to use a monolithic polishing pad.
Thus there is a need for a CMP apparatus, and method thereof, that provides optical access to the wafer front surface for continuous in-situ process monitoring, without undue process complexity or expense.